Quantum State Tomography of an Itinerant Squeezed Microwave Field
F. Mallet, Manuel A. Castellanos-Beltran, H. S. Ku, Scott Glancy, Emanuel H. Knill, K. D. Irwin, Gene C Hilton, Leila R. Vale, Konrad W. Lehnert · Physical Review Letters · 2011
We perform state tomography of an itinerant squeezed state of the microwave field prepared by a Josephson parametric amplifier (JPA). We use a second JPA as a preamplifier to improve the quantum efficiency of the field quadrature measurement from 2% to 36%±4%. Without correcting for the detection inefficiency we observe a minimum quadrature variance which is 68(-7)(+9)% of the variance of the vacuum. We reconstruct the state's density matrix by a maximum likelihood method and infer that the squeezed state has a minimum variance less than 40% of the vacuum, with uncertainty mostly caused by calibration systematics.